The invention described herein relates to a method and apparatus for axially adjusting the elevation of fuel rods in a nuclear reactor fuel assembly.
A conventional fuel assembly of the type used in commercial nuclear reactors which generate electric power, consists of a multiplicity of fuel rods held in spaced relation with each other by a series of grids spaced along the fuel assembly length. Top and bottom nozzles which lend support and rigidity to the assembly are then attached to the opposite ends thereof to provide a complete fuel assembly ready for reactor use. During the time that fuel rods are being pulled into aligned openings or cells in the spaced grids, springs in each cell force the fuel rod against projections or dimples formed in the wall of each cell. This action results in all fuel rods not being finally positioned in the assembly at the same level, i.e., their ends do not lie in the same plane. Also, this action occasionally causes some of the zircalloy material of the fuel rod to be scraped off on one or both projections. As the fuel rod continues to be pulled into each cell, the zircalloy material rolls into a ball and comes to rest on top of one or both projections. The result of material removal is that the fuel rod loses its center-to-center spacing with respect to other fuel rods to the extent of removal of material. Also, the scraped zircalloy material must be removed to preclude its circulation in the coolant system during reactor operation.
In the past, the fuel rod was moved slightly out of the grid to expose the scraped material and permit its removal manually. The rod was then returned to its original position. To move the fuel rod axially in the grid to not only allow access to the scraped material but also to locate the fuel rod at a precise level in a fuel assembly, a bar was inserted between the fuel rod and a nozzle plate to move the rod in a desired amount and direction. The bar exerted a force on the end of the fuel rod, but these forces were never applied uniformly in an axial direction. Since they were applied at a tangent to the rod axis, distortion of the fuel rod occurred with the result that fuel pellets therein were chipped or cracked and the pellet therefore lost its integral status, which in turn, could cause deviations in reactor performance. It therefore is apparent that the need exists for improved tools and procedures which will allow for adjustment of fuel rods in a fuel assembly to have them all appear at the same level while simultaneously permitting access to scraped material on a fuel rod.